In the prior art, LGA or BGA semiconductor packages are connected to packaging boards by direct soldering or by means of movable pins held for vertical motion by leaf springs or coil springs. However, as the number of external connection terminals is increasing in current semiconductor packages tailored for higher performance and function, simultaneous connection using solder is deemed difficult from the standpoint of connection reliability. Also in conjunction with the speed-up of electric signals, the conventional leaf springs and coil springs can now interfere with the high speed signal transmission because they have an increased inductance component due to long distances of connection.
In view of the above-discussed problems, electric connectors in which a plurality of conductive elastomer elements penetrate through and are supported by an insulating substrate are recently under study. Silicone rubber compositions having metal powder compounded therein are advantageously used as the material of which the electric connectors are constructed. As the metal powder to be compounded in these silicone rubber compositions, silver powder is frequently used from the standpoints of resistance and cost. The silver powder is classified into a reduced silver powder obtained by reducing an aqueous silver nitrate solution with a reducing agent such as hydrazine, formaldehyde or ascorbic acid, an electrolytic silver powder obtained through electrolysis of an aqueous silver nitrate solution for depositing silver on a cathode, and an atomized silver powder obtained by heat melting silver at or above 1,000° C. and atomizing the silver melt into water or inert gas. The shape of these silver powders is divided into granular, flaky, dendritic and irregular shapes. In general, the granular silver powder has the tendency that particles agglomerate together, so that compounding of the silver powder in silicone rubber results in the inconsistency of resistance because the resistance value is likely to fluctuate depending on the dispersed state of silver particles. Thus, the granular silver powder is often used in combination with the flaky silver powder.
For the flaky silver powder, a certain manufacturing process is by grinding a silver powder while treating it with saturated or unsaturated higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, and oleic acid, metal soaps, higher aliphatic amines, polyethylene wax or the like. However, the flaky silver powder by such a process can retard the vulcanization of silicone rubber to which the powder is added. It is also known that the silver powder which has not been treated as above results in the inconsistency of resistance of silicone rubber to which the powder is added.
An electric connector obtained by molding such a conductive silicone rubber composition in a mold or the like has an inconsistent conduction resistance when used in the mounting of a semiconductor package. As a result, the operation of the semiconductor package becomes unstable. Further, if compression of the semiconductor package to the packaging board is repeated, the agglomerated structure and chain of the silver powder are disrupted, resulting in a drastic increase of conduction resistance to inhibit repeated use.